Multilayers of fatty acid salts prepared by the Langmuir-Blodgett technique on solid supports build structures that are well ordered in growth direction perpendicular to the surface of the substrate. Open questions remain concerning the arrangement of molecules within the layers and their stacking from layer to layer. We have investigated Pb and Ni stearate multilayers on an Si support by means of grazing-incidence diffraction using X-rays from a synchrotron source. For Pb stearate, six in-plane Bragg peaks (0 < Q(z) < 3.5 angstrom-1) were detected, whereas only one peak was found for Ni stearate. The intensity distributions along the surface normal (rod scan) were recorded at each Bragg peak for 0 < Q(z) < 0.24 angstrom-1. Based on the experimental data a structure model of the three-dimensional arrangement of molecules within the multilayer domains is proposed which prefers interlayer chain coupling between neighbouring monolayers.